The activity pattern of the moss Hennediella heimii (Hedw.) Zand. was monitored over a period of 18 days during the austral summer season 2000/2001 at the Canada Flush in Taylor Valley, continental Antarctica. Provided with melt water from the massive Canada Glacier, the moss showed a constant potential photosynthetic activity during the entire measurement period. Permanently hydrated, the moss faced high light levels at surprisingly low moss temperatures, which is commonly supposed a deleterious situation for plants. The electron transport rate response of the moss to photosynthetic photon flux densities was linear at all temperatures and did not show a sign of saturation or photoinhibition. H. heimii seems to be well adapted to its environment and tolerates the ambient conditions without apparent harm. This might be due to the fact that mosses can acclimatise to high light conditions by building up highly effective non-photochemical quenching systems.